This invention relates generally to electronic test instruments and in particular to a test instrument adapted to automatically test semiconductor diodes by displaying both the actual diode orientation relative to the test probes and the forward bias junction voltage.
Semiconductor diodes are typically constructed of silicon or germanium material with two dissimilarly doped regions abutting against each other to form a junction. This junction has the property of allowing the conduction of electrical current in one direction and not the other. Diodes are used in a wide variety of electrical and electronic applications and play a number of different roles, including rectifiers in power supplies, switching elements in high frequency circuits, voltage references, and detectors. While the construction of diodes varies as widely as their applications, their common feature is in allowing current flow in one direction as a unipolar device. Test instruments adapted for testing diodes seek to exploit this unipolar feature in providing information on the condition of a diode during service and troubleshooting operations. The terminals of the diode are labeled as the anode and the cathode. The physical orientation of axial-leaded diode is typically indicated with the cathode as the banded end. However, diodes are often unmarked and sometimes mismarked, and the test instrument must be able to resolve the correct orientation of the diode relative to the test probes.
Curve tracers are typically large, sophisticated instruments designed for the laboratory environment to precisely characterize semiconductor devices over a continuous range of voltage and current levels. Curve tracers commonly provide for a bipolar test signal in the form of a swept current or voltage that sweeps from a negative to a positive voltage while the instrument monitors the desired device response and traces a curve on a display for the user. In this way, a complete diode curve including both forward and reverse orientations may be traced out to provide detailed information on the diode resistance, forward junction voltage, and reverse breakdown voltage, among other parameters. Such information is important to electronic designers. However, the curve tracer is ill suited for a troubleshooting environment in which the technician only wishes to know whether the diode is "good" or "bad" as well as the diode orientation relative to the test probes.
A digital multimeter that includes a resistance measuring function is the most common troubleshooting tool. Testing a diode with the resistance measuring function involves measuring the resistance across the diode first in one direction and then in the other. The unipolar nature of the diode provides for a relatively low resistance in one direction and a relatively high resistance in the other. A more meaningful indicator of a condition of the diode is its forward bias junction voltage. A diode constructed of various types of semiconductor material, such as silicon, tend to exhibit well-known voltage drops across the junction while the diode is biased in the forward direction. Silicon diodes tend to have a forward bias junction voltage in the range of 0.55 to 0.6 volts. Consequently, instrument manufacturers have implemented a diode test function in their digital multimeters that provides a measure of the forward bias junction voltage by coupling a d.c. (direct current) voltage source across the diode in the forward bias polarity and measuring the voltage drop across the diode.
A failed diode most commonly resembles either an open circuit or a short circuit, which are conditions relatively easy to detect with a multimeter. A "good" diode is one that conducts current in the forward direction only and assumes a forward bias junction voltage within an anticipated voltage range. A reverse-biased diode resembles an open circuit with no current flow. Evaluating diode orientation requires knowledge of the direction of the current flow through the test probes which are typically supplied in red and black colors to indicate a predetermined polarity. According to standard industry convention, the black test probe is placed on the anode and the red test probe is placed on the cathode to obtain the proper polarity to forward bias the diode and thereby obtain its forward bias junction voltage on a digital multimeter. The user must still manually probe the diode in both orientations in order to fully evaluate the function of the diode in both polarities.
Therefore, it would be desirable to provide a portable test instrument that has the capability of evaluating a diode in both directions, with no need for the user to manually reverse directions with the test probes, to supply information to the user regarding the diode forward junction voltage and the orientation of the diode relative to the test probes if the diode is deemed to be good and otherwise to supply information on whether the diode is open or shorted. In cases where there is no clear bipolar nature of the device being tested, such as a resistor that conducts current equally in either direction, the instrument simply returns information to the effect that the device is unknown.